In natural mineral oil deposits, mineral oil is present in the cavities of porous reservoir rocks which are closed off from the earth's surface by impermeable covering layers. The cavities may be very fine cavities, capillaries, pores or the like. Fine pore necks can have, for example, a diameter of only about 1 μm. In addition to mineral oil, including natural gas fractions, the deposits comprise water having a higher or lower salt content.
In mineral oil production, a distinction is made between primary, secondary and tertiary production.
In primary production, after sinking of the well into the deposit, the mineral oil flows by itself through the well to the surface owing to the autogenous pressure of the deposit. However, in general only from about 5 to 10% of the amount of mineral oil present in the deposit, depending on the type of deposit, can be extracted by means of primary production, after which the autogenous pressure is no longer sufficient for extraction.
Secondary production is therefore used after the primary production. In secondary production, further wells are drilled into the mineral oil-carrying formation, in addition to the wells which serve for production of the mineral oil, the so-called production wells. Water and/or steam is forced into the deposit through these so-called injection wells in order to maintain or to increase again the pressure. By forcing in the water, the mineral oil is forced slowly through the cavities in the formation, starting from the injection well, in the direction of the production well. However, this functions only as long as the cavities are completely filled with oil and the water pushes the more viscous oil in front of it. As soon as the low-viscosity water penetrates through cavities, it flows from this time on along the path of least resistance, i.e. through the resulting channel between the injection wells and the production wells, and no longer pushes the oil in front of it. As a rule, only from about 30 to 35% of the amount of mineral oil present in the deposit can be extracted by means of primary and secondary production.
It is known that the mineral oil yield can be further increased by tertiary oil production measures. Tertiary mineral oil production includes processes in which suitable chemicals are used as assistants for oil production. These include the so-called “polymer flooding”. In polymer flooding, an aqueous solution of a polymer having a thickening effect is forced instead of water through injection wells into the mineral oil deposit. By forcing in the polymer solution, the mineral oil is forced through said cavities in the formation, starting from the injection well, in the direction of the production well, and the mineral oil is finally extracted via the production well. Owing to the high viscosity of the polymer solution, which is adapted to the viscosity of the mineral oil, the polymer solution can no longer, or at least not so easily, break through cavities as is the case with pure water.
As an alternative approach, water-in-oil macroemulsions have been proposed as a method for producing highly viscous drive fluids that can maintain effective mobility control while displacing moderately viscous oils. For example, the use of water-in-oil and oil-in-water macroemulsions have been evaluated as drive fluids to improve oil recovery of viscous oils. Such emulsions have been created by addition of sodium hydroxide to acidic crude oils. In particular, U.S. Pat. No. 5,927,404 and U.S. Pat. No. 6,068,054 describe oil-in-water and water-in-oil emulsions that are stabilized by solid particles. These emulsions can be used to displace oil in subterranean formations.
U.S. Pat. No. 6,988,550 discloses a method to prepare an oil-in-water emulsion in a subterranean formation in the presence of hydrophilic particles such as bentonite clay and kaolinite clay both of which comprise negatively charged layers and cations in the interlayer spaces.
However, a more economic approach is to form an oil-in-water emulsion containing solid particles in situ in the subterranean oil-containing formation, recover the oil-in-water emulsion and separate off the different components so that the solid particles can be reused.